std::allocator_arg_t
From cppreference.com
                    
                                        
                    
                    
                                                            
                    |   Defined in header <memory>
   | 
||
|   struct allocator_arg_t {};  | 
(since C++11) | |
std::allocator_arg_t is an empty class type used to disambiguate the overloads of constructors and member functions of allocator-aware objects, including std::tuple, std::function, std::promise, and std::packaged_task.
[edit] See also
|    (C++11)  | 
   an object of type std::allocator_arg_t used to select allocator-aware constructors  (constant)  | 
|    (C++11)  | 
   checks if the specified type supports uses-allocator construction   (class template)  |